The present invention relates to a method and an apparatus for sampling electrical signals.
In fields such as radar engineering and telecommunication, information exists in the form of carrier frequency signals. In order to extract the information the signal must be demodulated, which is most often achieved by down-mixing to the frequency 0, so-called baseband or "zero-IF" systems. Often the down-mixing is performed in two steps, first from high frequency to an intermediate frequency and thereafter to the baseband.
In the above systems, the information exists both in the amplitude of the signal and in its phase. A simple envelope detection is therefore not sufficient. For this reason the last down-mixing is performed in two mixers in which one of them is supplied with the intermediate frequency signal and a reference signal and the second with the intermediate frequency signal and the reference signal phase-shifted 90.degree.. The frequency of the reference signal is then equal to the centre frequency of the intermediate frequency.
The result is two orthogonal signals which can be regarded as a complex representation of the baseband signal in which both amplitude and phase information are preserved. As a rule, these signals are called I- (In-phase) and Q(Quadrature) signals.
As modern signal processing almost exclusively uses digital technology, the I- and Q-signals are converted into digital form in an analogue/digital converter, after which the signal processing, for example filtering, is performed.
In many applications, and especially in radar applications, the requirements for amplitude equality and phase orthogonality between the I- and Q-signals are very high. This implies that some kind of automatic, active calibration procedure usually is used. The calibration procedure consumes signal processing capacity and also requires certain additional equipment. It is therefore desirable to eliminate the need for calibration.
One method of eliminating the calibration need is to perform the analogue/digital conversion directly on the inter-mediate frequency signal and thereafter split up the obtained signal samples into one I- and one Q-value by means of digital filters. Examples of apparatus which utilize this method are described in the patent documents EP-109121 and UK2176362.
Even this method has drawbacks. The use of the method is thus limited due to limitations in the conversion rate of the analogue/digital converters in combination with accuracy. The requirements as regards rate for the logics in the digital filters are also high. The rate requirements lead to an increased power requirement.
The object of the now present invention is therefore to eliminate the need for filtering in order to provide quadrature signals from an amplitude modulated carrier signal and to reduce the requirements on the analogue/digital conversion rate at the same time as high requirements are met regarding accuracy in the I- and Q-channels and very low power consumption.